1. Field of the Invention
The present invention relates to an apparatus for protecting electrical circuits from the effects of component product residue due to circuit failure. More particularly, it relates to an apparatus for protecting potted electrical circuits enclosed within housings that may typically employ surge suppression or other like circuitry used in the mitigation of electrical anomalies and abnormal conditions in devices to which the surge suppression circuitry is connected from component product residue that may be subjected upon surrounding circuitry when a failure of any particular circuit occurs.
2. Description of the Prior Art
Component failure on circuit boards of all types is very well known in the prior art. Some component failure can have little or no effects on the circuit itself and/or the surrounding circuitry. However, in other circuitry, component failure can have devastating effects on the specific circuit board containing the failed component or on a surrounding circuit board, circuit panel or other electrical equipment in close proximity thereto.
Circuitry that is specifically designed to fail in certain circumstances can be found in Transient Voltage Surge Suppression (TVSS) devices, also known as Surge Protective Devices (SPD). These devices are well known in the prior art. TVSS devices are used with computers, other types of electrical equipment and electrical circuits to protect against electrical line voltage surges and other occurring transient electrical anomalies that may occur along an electrical power line to which the equipment or circuit is coupled. Transient over voltages or voltage surges result in peak voltage levels that occur within an electrical line that are higher than that which the equipment being protected is rated to handle (i.e., over-voltages). Transient electrical anomalies include other types of electrical occurrences in the electrical line that are considered sub-cycle events which cause the equipment to operate improperly or completely fail (i.e., over-currents and the like). Transient over voltages or voltage surges have become even more problematic in today's sophisticated electrical world wherein sensitive computerized equipment requires a constant and regulated supply voltage.
Various types of voltage surges that can affect an electrical device or circuit can occur at any given time within an electrical line, and include Temporary Over Voltages, and abnormal conditions such as Full-Phase Over voltages and Limited Current Over voltages. It is desirable to suppress any excess voltage or current that occurs on an electrical circuit that has been disturbed. Studies by the Institute of Electrical and Electronics Engineers (IEEE) have shown 120 Volt power lines reaching as high as 5,600 Volts. Suppression of these over-voltages and other transient electrical anomalies is highly desirable, and many instances critical, to the operation of an electrical device, computer or electrical circuit.
Transient over voltages and voltage surges can occur due to a plurality of factors, to include, but are not limited to, internal anomalies and external anomalies such as, lightning strikes, circuit overloads, power company grid activity, temporary or permanent failure of the neutral conductor, recovery from brown-outs, black-outs and circuit interruption by simple human error. Since transient over voltages and voltage surges are very common, but difficult to predict, it is imperative to have a TVSS device on line at all times protecting the targeted piece of electrical equipment or circuit. To leave the equipment unprotected can result in a devastating consequence wherein the equipment is temporarily brought off-line, severely damaged or completely destroyed. This result could be detrimental to hospitals, police, fire and rescue units, the military and other critical functioning entities who can not afford to be “off-line” for any period of time. It is therefore critical to suppress these surges and anomalies and minimize their effect on the electrical equipment they are charged to protect.
It is understandable however, that even the best, most complex, most redundant TVSS devices are not one hundred percent effective against all electrical line surges and transient anomalies. However, suppressing as many of these surges and transients as possible most likely will result in a longer life for the electrical equipment that the TVSS device is protecting. It will also help to minimize any possible “down-time.”
Many TVSS devices, and in particular solid state devices, employ metal oxide varistors (MOVs) to provide for a non-linear voltage-current relationship for handling the surge suppression. However, other TVSS devices exist, which employ silicon avalanche diodes (SADs), zener diodes, selenium cells and high voltage capacitors for surge suppression. In other TVSS devices, gas discharge tubes are employed for the surge suppression component. MOV designed TVSS devices are favored over many other surge suppression components due to their ability to be used in low voltage applications, such as, for example, AC power distribution systems having a normal and nominal operating voltage less than or about 600 volts AC (600 VAC).
MOVs act as a type of current diverter for the TVSS device. Under normal conditions, TVSS device surge suppression components, such as MOVs, draw very little current. As the voltage level increases across the TVSS device, to a level higher than the system voltage and that which the equipment it is protecting is designed (rated) to handle, the impedance of the TVSS circuit drops significantly, effectively causing electrical conduction across the surge suppression components or MOVs. Since very low impedance is required in voltage surge suppression, this result is highly desirable. The result of this current diverting scheme is voltage surge absorption by the MOVs. The energy absorbed by the surge suppression components, such as the MOVs, is dissipated as heat. In some instances (a sustained over voltage condition), the heat rises to a level which causes the MOV, or other surge suppression component, to burn, melt or explode. Although the desired result of voltage surge suppression may have been realized, the melting, burning or explosion of the MOV can cause other problems that must be addressed. For instance, the MOV can vaporize, which can result in plasma being formed, which in turn creates a new electrical conductor. This new electrical conduction defeats the purpose of the TVSS device resulting in damage to the equipment that the TVSS device is meant to protect by letting through the voltage surge or transient which the TVSS was intended to suppress. Further, unacceptable end of life conditions can cause damage to the equipment that the TVSS was intended to protect as well as surrounding equipment near the installation site of the TVSS. For these reasons, it is imperative to provide a mechanism for the TVSS device which would permit it to operate up to a level which provides for adequate transient voltage surge suppression all the while providing for the ability to be brought off line to avoid TVSS device failure or environmental hazards in cases where the surge protective device components are reaching their end of life. In other words, it is desirable to provide a manner in which the TVSS device, or other like devices, can safely fail, otherwise known as “acceptable failure modes” or “acceptable end-of-life conditions.” And in those situations wherein acceptable failure modes or acceptable end-of-life conditions results in component product residue being projected and thereby subjected on surrounding circuitry, it is desirable to contain such residue and minimize or eliminate damage to the surrounding circuitry.
To avoid the problems associated with surge suppression component (i.e., MOV) destruction, which can cause TVSS device failure, advancements have been made to TVSS devices, including but not limited to, device container improvements to quell or contain plasma formation and the provision of fusing circuitry to prohibit MOV burn-up. U.S. Pat. No. 5,488,534 to Rau et al. addresses both of these issues. To inhibit plasma formation, a plastic cover and housing are used for the TVSS module, silver wires serve as fuses or fuse links since they are known to have a lower oxidation energy level than copper or aluminum, and longer lengths are used for the fuse wires which raises voltage re-strike levels thereby reducing arc formation that may occur in a wire melt down situation. In this prior art device, one wire fuse is used for every MOV that is employed. Although this may be appropriate when employing small thermal fuses for each MOV, this would be more difficult if using larger current rated fuses. If so, the use of one current rated fuse for every MOV would require that a larger housing be employed. For those circuits that require a significant level of transient voltage surge suppression, the size of such a device would not be acceptable for most applications with today's modern need for limited space requirements. Further, even if the TVSS device could be made to be compact due to a minimal amount of surge suppression components being employed, and therefore a minimal number of fuses being employed, space requirements within the environment where the device is employed could still dictate that the device of this prior art reference is too large (i.e., the confined spacing requirements of a naval vessel). Still further, the use of a current rated fuse for every MOV would most likely cause the TVSS device to constantly go “off-line” due to the fast acting response time of standard current rated fuses. This of course would defeat the entire purpose of the TVSS device. On the other hand, if only thermal fuses are employed, then the problems associated with a non-performing TVSS device that has reached its end of life due to high current abnormal over voltage conditions, will still exist since the thermal fuses only work at the component level to take over-heated MOV components off-line. Simply put, the use of thermal fuses only at the component level is inadequate for today's needs in transient voltage surge suppression technology. Another aspect of this prior art reference, as to the issue of containment of component product reside, relates to the use of a sand and epoxy fill material, which is used to fill spaces and voids to prevent the spread of any plasma created due to component failure or fuse wire clearing. However, the use of such fill material is very limiting in that once it is applied to the circuit it can not be removed to change simple device repairs.
Other attempted advancements in the prior art of improved safety for surge suppression circuits can be seen in European Patent Application No. 07250027.5 (Publication No. EP 1 806 818 A2), wherein thermal cutoffs are employed and revised layouts or orientations of the components are chosen to reduce the risk of burning through the outer enclosure in the event that electrical components, such as MOVs, over heat. However, revised layouts and/or orientation of components can limit what can be placed on a specific circuit board and is not an ideal way to solve the problem of component product residue containment.
Other attempted advancements in the prior art, such as U.S. Pat. No. 6,184,464 to Liptak et al., have placed restrictive and/or resilient materials around a circuit board before it is potted with a material, which is poured over the circuit board and the restrictive and/or resilient material and then allowed to hardened. Although this has provided an added protective layer, it again is limiting in that no modifications to the device, circuit board or material layers can be made once it is encapsulated within the potting material.
In view of all of the above, improvements are clearly needed in the art of protecting circuit boards and devices in general, which employ circuitry that is known to, or designed to purposefully overheat or fail upon certain events occurring, like those most often seen in surge suppression and like devices.